The present invention is directed to a device for measuring the thickness of a film applied on a substrate without contacting the film itself. In particular, the device may be used in combination with a spray machine for measuring the thickness of a wet or dry paint film sprayed on a metal substrate, and which controls the thickness of the paint film to be applied to the next substrate.
In the spray application of many paints, a relatively thin film is formed near the edge of the article being coated and is substantially thicker in the center of the article. This difference in thickness of the paint film results in appearance changes from the center to the edge of the article. This is a problem in the coating of articles, such as auto or truck body panels, in which a uniform appearance is desired, and in particular is a problem for paint panels which are used as color standards in laboratories, paint manufacturing plants and automotive and truck assembly plants. In particular, rejects of paint panels to be used for color standards can be as high as 50% of the panels coated for such uses.
An example of a spray machine is shown in Falcoff, U.S. Pat. No. 4,614,300, issued Sept. 30, 1986. Many spray parameters are set and automatically controlled by this machine with the exception of the paint film thickness.
Another example of an improved paint spraying device with wet film thickness measurement and feedback control is shown in Falcoff, U.S. Pat. No. 4,702,931, issued Oct. 27, 1987. However, this device has the disadvantage that the sensors for measuring the distance from the device to the article being coated are disposed on opposite sides of the substrate. This severely limits the use of this device in an assembly line work where series of articles to be coated are generally conveyed in succession. In particular, where the articles being painted are auto or truck body panels, it is virtually impossible to position the sensors on both sides of the panels.
Another device for measuring thickness of coatings on substrates is disclosed in "Measuring Thicknesses of Coatings on Metals," NASA Tech Briefs, May/June 1986, pages 94 & 96. This reference discloses the use of a triangulation optical sensing arrangement by using a helium-neon laser with an angle sensing photo-detector in a side-by-side configuration with an inductive proximity sensor. However, this arrangement has two disadvantages. First, since the pair of sensors are disposed side-by-side, the angular orientation to the substrate surface must be precise in order to obtain reproducible results. A slight angular misalignment causes the distance from each sensor to the surface to change, resulting in an incorrect coating thickness measurement. Second, the optical sensor, as described in the reference, utilizes a beam of light normal to the surface and detects light from the surface at an angle. Accordingly, the performance of the sensor is dependent on the reflectivity and scattering properties of the film surface. Since some scattered light must be present for the optical detector to function, a perfect reflecting surface would not be measurable with this configuration. For example, a glossy surface, such as that found in automotive finishes, might be difficult to measure, particularly in bright ambient light.
In view of the above, there is a need in the art for a non-contact wet or dry film thickness measuring device which does not have the disadvantages associated with the conventional devices, and which controls the thickness of the paint film applied on the substrates.